1. Field of the Invention
The present invention relates to an optical beam scanning device and more particularly to the configuration of a slit mechanism for receiving a scanning optical beam and for producing a horizontal synchronizing signal.
2. Description of the Related Art
Japanese Patent Application Publication Kokai No. HEI-5-19194 describes an optical beam scanning unit (optical beam scanning device) for use in an electrophotographic image forming apparatus such as a laser printer, a copier, or a facsimile machine. The optical beam scanning unit includes: a laser unit for emitting a laser beam; a rotatable polygon mirror such as a hexagonal mirror for deflecting the laser beam; and a lens system for focusing the laser beam into a beam spot onto a subject medium thereby scanning the beam spot over the subject medium. These components are all encased in a box-shaped housing case.
The laser unit emits a laser beam modulated according to image data. The laser beam irradiates and reflects off the rotating polygon mirror. The reflected laser beam passes through the lens system and passes through a long and narrow window formed in one side of the housing case and converges on the surface of the medium to be scanned. The incident laser beam scanningly irradiates the medium in one scanning line by one surface of the polygon mirror.
A write start beam detecting unit is provided to the optical beam scanning unit for detecting a write start timing at which writing with the laser beam of one scanning line starts on the subject medium. The write start beam detecting unit includes: a write start beam detection mirror; a slit body; an optical fiber; and a photosenser. While being deflected by one surface of the polygon mirror, the laser beam first reaches the write start beam detection mirror and then starts forming a scanning line on the subject medium. When reaching the write start beam detection mirror, the laser beam reflects off the write start beam detection mirror and then passes through a thin slit formed in the slit body to fall incident on the optical fiber. The laser beam is guided by the optical fiber to the photosensor which is positioned at the opposite end of the optical fiber. Upon receiving the laser beam, the photosensor produces a write start beam detection signal. The laser unit will be modulated by the image data in synchronization with the write start beam detection signal. Thus, the write start beam detection signal is used as a horizontal synchronization signal.
The slit body is constructed from a pair of light-obstructing walls that form the left and right sides of the slit. The light-obstructing walls have planer surfaces orthogonal to the optical axis of the laser beam reflected from the write start beam detection mirror. Light directed toward and incident on the slit body partly reflects off the planer surfaces of the light-obstructing walls back to the polygon mirror via the write start beam detection mirror. After reflecting off of and being deflected by the polygon mirror, the laser beam will travel through the lens system to irradiate the surface of the medium to be scanned. Because this light converges on the surface of the medium, undesired ghost images such as black lines are formed in the image forming region of the subject medium.
To prevent interference caused when light reflected off of the write start beam detection mirror toward the photosensor intersects in the housing case with scanning light for forming an image on the surface of the subject medium, or when the scanning light reflects off the slit body or the photosensor, the housing case must be made large to secure space for the write start beam detection mirror and the photosensor.
In order to adjust the write start timing, the slit body must be moved in a direction normal to the optical axis of the laser beam reflected from the write start beam detection mirror, i.e., in a direction parallel to the light-obstructing wall surface. However, thus moving the slit body in a direction normal to the optical axis of laser light requires a large housing case with space enough for such movement. A large housing case, however, is undesirable.
In order to move the slit body in the direction orthogonal to the optical axis of laser light from the write start beam detection mirror, a contrivance such as forming a guide groove parallel to the direction of movement is necessary. Even when such a contrivance is provided, minute adjustments are still difficult.